1) Field of the Invention
The present invention relates to a repeater and a Raman amplification repeating system.
2) Description of the Related Art
The needs of international communications have been on the expansion at a rapid rate in conjunction with the globalization of business, the spread of the internet, and the like. In this situation, the submarine optical transmission system occupies an important position along with satellite communications and there is a pressing need to accomplish the early realization of an economical large-capacity system.
In such a submarine optical transmission system, an optical fiber cable is laid on the sea bottom and a repeater(s) is provided in the middle of the optical fiber cable to make optical transmission through optical amplification. Moreover, the submarine optical transmission system requires the highest reliability, for that a trouble of its in-the-sea portion requires a large amount of cost and takes time for repair. Therefore, there is a need for the system to have a trouble detection function to, when a trouble occurs by some change or other, detect the trouble site precisely.
For the trouble detection control, a terminal station on land first transmits a monitor/control command signal to each of repeaters for monitoring and controlling an operating state of each of the repeaters and, when receiving the monitor/control command signal therefrom, the repeater monitors its own operating state and transmits answer information indicative of a monitor result to the terminal station. In this way, a state of submarine optical transmission is monitored through communications between the terminal station and the repeaters.
Meanwhile, a conventional repeater employs EDFA (Erbium-Doped Fiber Amplifier) for optical amplification. that is, this EDFA type repeater is designed to superimpose an answer signal on a main optical signal and, in this case, the output of an excitation laser diode, which excites the EDFA, is modulated with the answer information to modulate the main optical signal for the superimposition of the answer signal on the main optical signal.
In addition, even if a disconnection/break trouble occurs in an optical fiber cable so that the optical main signal itself disappears, the EDFA repeater can transmit an answer signal by modulating an ASE (Amplified Spontaneous Emission) light from the EDFA itself serving as an amplification medium, thereby avoiding the incapability of the monitor control.
On the other hand, in the recent optical communication systems, attention is paid to an optical fiber amplifier utilizing a non-linear optical phenomenon in an optical fiber, so-called Raman amplification. The optical fiber amplifier achieves the optical amplification by making an intensive exciting light incident on the entire optical fiber transmission line, utilizing a physical phenomenon in which light different in wavelength from incident light scatters (Raman Scattering) due to a vibration phenomenon in a substance.
When such a Raman amplification method is used for a repeater to accomplish the optical amplification, larger-capacity transmission becomes feasible, in comparison with a conventional method. The Raman amplification repeater employing this Raman amplification transmits a response signal indicative of an operating state of the Raman amplification repeater according to a monitor/control command. As in the case of the EDFA repeater, the Raman amplification repeater modulates the output of an excitation laser diode, which is for exciting an optical fiber acting as an amplification medium, with answer information. It follows that the Raman amplification repeater modulates the optical main signal with the answer information, and thereby transmits an answer signal to the terminal station side.
Incidentally, although the technique on the Raman amplification repeat is disclosed in two documents: Japanese Laid-Open No. HEI 01-217424 and No. 2000-98433, these documents does not touch the technique of monitoring a state of optical transmission on the sea bottom through communications between a terminal station and a repeater(s).
However, in the case of the above-mentioned Raman amplification repeating, if a disconnection/break trouble occurs in an optical fiber cable in the vicinity of the Raman amplifier repeater, the amplification medium disappears and, hence, no emission of the ASE light occurs. Accordingly, difficulty is experienced in transmitting an answer signal to a terminal station, which affects the monitor control.
Moreover, in a case in which an SV signal serving as a monitor control signal is repeated together with the repeating of an optical signal in making the repeating transmission in the Raman amplification repeater, there is a need to provide a technique of transmitting the SV signal even if an optical signal acting as a main signal is not transmitted due to the fiber disconnection or the like in the former repeating zone.